Although nuclear medicine was traditionally limited to studies of organs exhibiting little internal motion, advances in the state of the art have made the collection of data related to heart function a common procedure today. Nonetheless, irregularities in a patient's heart rate (even where such patient is relatively immobile, as is the case during a nuclear study) makes the real time acquisition of reliable cardiac data a difficulty.
In conventional nuclear imaging of relatively immobile body parts, information accumulates at a relatively slow rate so the study is simply continued until a sufficient amount of data is collected to satisfy the requisite diagnostic needs. When an organ exhibiting a cyclical motion is studied, however, each cycle of the organ's motion must be separated into a finite number of segments and corresponding segments from a plurality of data cycles added (typically by a digital computer) so that each segment of a composite cycle has sufficient data to reproduce the organ in motion.
In cardiac imaging, the heart cycle is typically broken into, for example, 16 equally spaced intervals of time with the data representative of each of said intervals directed to selected locations within an imaging memory to help format the data for subsequent visual inspection and analysis. In a conventional cardiac study, the goal is to acquire sufficient data to reconstruct the various phases of the heart. This requires the inclusion of only that data which contributes to a regular heart cycle. However, the length of the heart cycle for a given patient, even at rest, may vary due to abnormalities in that patient, e.g., arrhythmia. Thus, a problem arises in the data collection since all of the heart beats in a given study may not lend themselves to an identical number of equally spaced intervals. Typically, the beginning of a cardiac cycle, for example as detected by an electrocardiogram (ECG), is used to trigger the start of a data acquisition cycle. Thus, if the heart beats at an irregular interval, the composite image formed by adding data from like numbered intervals of a series of cycles will be error filled. Some of the data will be missing as a result of restarting the data collection cycle before the system has completed the previous cycle. Other data will be included, but not synchronized, thereby adding further error to the final result.
There are two known approaches to radionuclide cardiac imaging. The first is to segment the cardiac cycle into a preselected number of equally spaced intervals and to start the data acquisition at the beginning of each cycle. The problem with this technique, as pointed out above, is that if the patient's heart beats prematurely during any cycle, the data collected in each interval during that cycle will be out of phase with the true cardiac data since the original interval calculation is based on the normal heart rate. The erroneous data cannot be removed from the study since they are already added into the cumulative data for each interval by the time the system can recognize that the cycle represents a shortened heartbeat. A similar problem occurs for each slow heartbeat.
The alternative is to collect all of the data generated by the camera system along with timing information that will permit the system to post-process the data acquired during the study and to reconstruct the heart in its various phases based only on the data collected in the heartbeats of interest. This is the technique currently used to overcome the limitations inherent in the variable heart rate; however, the technique obviously requires a great deal of on-line storage capability to contain all of the data and timing information. In addition, all of that data must be scanned and reformatted into the images of interest, adding delay to image formation.